This invention relates to an optical deflector used for a laser beam printer.
In general, an optical deflector is provided with a motor to rotate a polygonal mirror for deflecting a laser beam emitted by a laser generating device sush as a semiconductor laser diode. In such a motor, friction between a motor shaft and bearings needs be minimized for high speed rotation. To meet this requirement, the motor shaft is radially supported by journal bearings of a dynamic pressure type, and is floated or suspended to be axially supported by a magnetic thrust bearing.
In the motor mentioned above, air is introduced into a gap of 3 .mu.m to 6 .mu.m between the bearing member of dynamic journal bearings of herringbone type through herringbone grooves. As a result, a bearing pressure within the gap is increased at the center portion of the bearings, so that the motor shaft is subjected to a radial force. The motor shaft is suspended in a position in which the thrust load and the magnetic force of the thrust bearing are balanced. This rotary mechanism with the axial bearings and the thrust bearing permits the motor to rotate at high speeds in the range of 4,000 rpm to 15,000 rpm.
In a conventional arrangement of this type, however, the motor shaft is liable to be magnetized in the machining process and allow ingress of iron particles which may interfere with the preferable rotation of the motor shaft. When the shaft is locally magnetized, the shaft suspended by the magnetic thrust bearing is effected by the local magnetization. The local magnetization causes relative eccentricity of the shaft against the journal bearings which may result in contact of the shaft and the bearings, and faulty rotation.